Tunable Dual-Passband Microwave Photonic Filter Using Orthogonal Polarization Modulation

A microwave photonic filter (MPF) with two independently tunable passbands using a polarization modulator (PolM) and a phase-shifted fiber Bragg grating (PS-FBG) is proposed and experimentally demonstrated. By applying two orthogonally polarized optical waves from two tunable laser sources (TLSs) to the PolM, two phase-modulated optical signals that are orthogonally polarized are generated, which are sent to the PS-FBG that acts as an optical notch filter to suppress one sideband of the orthogonally polarized phase-modulated signals. Thus, the two phase-modulated signals are converted to two intensity-modulated single-sideband signals. The overall operation corresponds to a dual-passband MPF with the central frequencies of the two passbands independently tunable by changing the wavelengths of the optical carriers from the two TLSs. The proposed MPF is experimentally evaluated. A dual-passband MPF with a bandwidth for each passband of 140 MHz and a frequency tunable range of ~6 GHz is demonstrated. The insertion loss and the dynamic range of the filter are also studied and measured experimentally.